1. Field of the Invention
The present invention relates to a biaxially-oriented styrene based resin sheet, and in particular to a multilayered biaxially-oriented styrene based resin sheet which can be formed under a wide range of heat conditions and which enables improvements in deep drawability and form reproducibility by secondary forming and the suppression of raindrop formation without losing the excellent transparency and durability properties of an OPS sheet.
2. Description of the Related Art
Biaxially-oriented polystyrene sheets (hereafter referred to as OPS sheets) are widely used in packaging containers for light weight foods and in the packaging of other products as they display excellent properties of environmental hygiene, nerve, transparency, mold reproducibility and recoverability. Common secondary forming method of the sheets includes the so-called contact heating pressure forming method wherein the sheet and a hot plate are convection heated and the softened sheet is then pushed into a mold cavity by pressure from the hot plate and shaped. In this method, by sandwiching the sheet between an external frame and the hot plate with excess load, orientation shrinkage is prevented, and because a large pressure is applied, this molding method is ideal for the forming of biaxially-oriented sheets which possess orientation. However, there were several problems with this secondary forming method including difficulty in controlling of the hot plate temperature during the early stages of secondary forming or upon alteration in the forming conditions, a worsening in the form reproducibility when the hot plate is at lower temperatures, and furthermore the occurrence of a spotted pattern known as raindrops in sections where the hot plate temperature was too high.
In order to overcome the problems described above, Japanese Patent Application, First Publication No. Hei-2-239933 discloses a technique to prevent the occurrence of raindrops associated with fluctuations in the hot plate temperature by secondary forming, by using a copolymer of styrene and methacrylic acid as the resin for the outer layer and polystyrene as the inner layer to provide a biaxially-oriented sheet with a wider forming temperature range.
However, although the biaxially-oriented sheet of Japanese Patent Application, First Publication No. Hei-2-239933 certainly increases the temperature at which raindrops occur by secondary forming and widens the forming temperature range, because the proportion of methacrylic acid in the resin of the outer layer is high, the difference in the fluidity of the resin of the outer layer and that of the inner layer is large, and stripe and wave patterns develop on the surface of the sheet, leading to a whitening and a large drop in the transparency and external appearance of the sheet. Particularly in large production devices with large dischargable volumes there is a tendency for the shearing stress value in the layer combining section to increase giving a marked deterioration in external appearance. Furthermore, because there is no compatibility between the resin of the outer layer and the resin of the inner layer, recovering scraps of the sheet during sheet production or during removal following sheet molding, recovering waste portions of its molding products, and then reusing the scraps in biaxially-oriented sheets also leads to whitening.
Furthermore, for biaxially-oriented sheets of the same layered construction described above, a technique for improving the fluidity and compatibility of the resins of the inner and outer layers by lowering the proportion of methacrylic acid incorporated in the outer layer is disclosed in Japanese Patent Application, First Publication No. Hei-7-156342, which proposes a biaxially-oriented sheet comprising an outer layer of a copolymer of styrene and methacrylic acid in which the weight proportion of methacrylic acid is in the range from 1 to 4%, and an inner layer of polystyrene, but although the external appearance of this sheet is good due to the good compatibility of the inner and outer layers, there is a narrowing of the forming temperature range, and the occurrence of raindrops by secondary forming becomes problematic.